Non-aqueous, semi-aqueous and aqueous suspensions of non-colloidal high density abrasive particles have been previously used in wire saw cutting and lapping of wafers but have been unsuccessful in obtaining a stable slurry. U.S. Pat. No. 5,099,820 issued to Stricot discloses an abrasive slurry of a suspension of silicon carbide particles in water or oil. However, the suspensions are not stable and do not provide uniform lubrication and cutting by the wires. Such compositions require vigorous agitation to maintain a uniform suspension of particles, and the suspensions settles out quickly under stagnant conditions even during work-piece slicing while still under agitation.
Patent application Ser. No. 09/637,263 filed Aug. 10, 2000 to Ward et al, now U.S. Pat. No. 6,602,834 which is herein incorporated by reference, discloses a non-aqueous or semi-aqueous cutting and lubricating composition for use with wire saws that relies upon a surfactant, a polyelectrolyte and pH to provide electrostatic repulsion and particle—particle interference to maintain a stable suspension of abrasive particles.
U.S. Pat. No. 6,054,422 to Ward et al discloses a lubricating composition containing up to 70 weight percent abrasive grit material in a suspension utilizing a mixture of high and low molecular weight polyalkylene glycols and a suspension agent.
In the production of Silicon, SiC, Sapphire, GaAs, optical glass, and other wafers used in a variety of industries (i.e., Microelectronics, Solar Cells, L.E.D.'s, broad band-width devices, optics/lasers, wafer polishing, CMP applications, many others) wafers are cut from larger ingots, bricks, boules, etc. The next step following the initial cut of the wafer, disc, piece, etc., involves the LAPPING of the cut wafer to smooth out the surface, lower the TTV, eliminate damage depth defects and prepare the wafer for final “POLISHING”. In general, aqueous carriers are used as the suspension media for the lapping abrasives employed in this step. Lapping abrasives can include, but are not limited to: SiC, Aluminum oxides, ZrO2, Silicas, diamond, etc. Lapping slurries utilize abrasive particles that are in the size range of about 0.5–20 μm. This means that the suspended abrasive particles are typically non-colloidal in size and nature. This does not exclude the use of colloidal lapping abrasive (i.e. abrasive particles of size range from about 0.001–1.0 um), but such particles are not typically used in lapping slurries.
The lapping slurry for wafers, gears, ceramic, etc., is subjected to many shear, grinding, and abrasive forces during the wafer lapping process. During the process of “planetary lapping”, the slurry is injected onto the wafer surface, which is held between two large iron plates. Counter rotation of the upper and lower plates holding the wafer compresses the slurry between the upper plate and the wafer surface. The solids within the compressed slurry contact the wafer, and angular momentum, causes the abrasive action to remove surface wafer defects and “etch” away the desired amount of wafer surface material. With all aqueous slurries used today in lapping, such action on the slurry and the design of the lapping equipment propagates particle agglomeration on the wafer, within the reservoir, within the feed piping, within the lapper, on the iron plates, etc. Such particle agglomeration has the added deleterious effect of producing damaging “scratches” on the lapped wafer. Such wafers must then be discarded at great cost.
Aqueous suspension of non-colloidal (i.e. NCOL), high-density abrasive particles has been a severe and debilitating problem for “wafer” manufacturers for several decades. To date, there exists no low viscosity, water-based carrier that will maintain “NCOL” abrasive particle suspension for more than an extremely short time period of a few to several minutes. After that, particles begin to agglomerate and settle out of suspension quickly to the bottom of the container. Such abrasive particle settling in current “aqueous” slurries occurs quickly, even during constant mixing or recirculation. This particle settling is typically manifested by a “hard settled cake” at the bottom of the container. Any attempt to regenerate slurry, which would maintain the original particle size distribution of the virgin abrasive, cannot be accomplished by simple mixing, agitation, shaking or the like. As a result, such slurries are immediately discarded, wasting expensive abrasive, time, manpower, and effort.
In prior art suspensions, temperature, and pH played a factor in the amount of time that a suspension remains homogenous and uniform in extended stagnant storage. Inorganic particles may remain in suspension in aqueous and non-aqueous solvents depending upon the size of the particle, lattice structure and density but in stagnant storage tend to agglomerate and settle out of suspension.